Add context hooks for metrics and sample manifest

[owtaylor]

It's inefficient that the metrics and sample manifest are reloaded every time the user navigates to those pages - we really should be able to just load them once when the page is loaded.

To fix this, move web/src/utils/fetchMetrics to web/src/context/Metrics.tsx and make it export:

• A hook 'useMetrics' that retrieves the metrics (or undefined before they are loaded)
• A component MetricsProvider

Similarly, extract the sample manifest reading code from web/src/components/Samples.tsx to web/src/context/SampleManifest.tsx and make it export useSampleManifest() and SampleManifestProvider.

Then use these in App.tsx so that all the pages have access to the context, and modify page components appropriately.